Oil sand is essentially comprised of a matrix of bitumen, solid mineral material and water.
The bitumen component of oil sand includes hydrocarbons which are typically quite viscous at normal in situ temperatures and which act as a binder for the other components of the oil sand. For example, bitumen has been defined by the United Nations Institute for Training and Research as a hydrocarbon with a viscosity greater than 104 mPa s (at deposit temperature) and a density greater than 1000 kg/m3 at 15.6 degrees Celsius.
The solid mineral material component of oil sand typically consists of sand, rock, silt and clay. Solid mineral material may be present in oil sand as coarse solid mineral material or fine solid mineral material. The accepted division between coarse solid mineral material and fine solid mineral material is typically a particle size of about 44 microns. Solid mineral material having a particle size greater than about 44 microns is typically considered to be coarse solid mineral material, while solid mineral material having a particle size less than about 44 microns is typically considered to be fine solid mineral material. Sand and rock are generally present in oil sand as coarse solid mineral material, while silt and clay are generally present in oil sand as fine solid mineral material.
A typical deposit of oil sand may contain (by weight) about 10 percent bitumen, up to about 6 percent water, with the remainder being comprised of solid mineral material, which may include a relatively small amount of impurities such as humic matter and heavy minerals.
Water based technologies are typically used to extract bitumen from oil sand ore originating from the Athabasca area in northeastern Alberta, Canada. A variety of water based technologies exist, including the Clark “hot water” process and a variety of other processes which may use hot water, warm water or cold water in association with a variety of different separation apparatus.
In a typical water based oil sand extraction process, the oil sand ore is first mixed with water to form an aqueous slurry. The slurry is then processed to release bitumen from within the oil sand matrix and prepare the bitumen for separation from the slurry, thereby providing a conditioned slurry. The conditioned slurry is then processed in one or more separation apparatus which promote the formation of a primary bitumen froth while rejecting coarse solid mineral material and much of the fine solid mineral material and water. The separation apparatus may also produce a middlings stream from which a secondary bitumen froth may be scavenged. This secondary bitumen froth may be added to the primary bitumen froth or may be kept separate from the primary bitumen froth.
A typical bitumen froth (comprising a primary bitumen froth and/or a secondary bitumen froth) may contain (by weight) about 60 percent bitumen, about 30 percent water and about 10 percent solid mineral material, wherein a large proportion of the solid mineral material is fine solid mineral material. The bitumen which is present in a typical bitumen froth is typically comprised of both non-asphaltenic material and asphaltenes.
This bitumen froth is typically subjected to a froth treatment process in order to reduce its solid mineral material and water concentration by separating the bitumen froth into a bitumen product and froth treatment tailings.
In a typical froth treatment process, the bitumen froth is diluted with a froth treatment diluent to provide a density gradient between the hydrocarbon phase and the water phase and to lower the viscosity of the hydrocarbon phase. The diluted bitumen froth is then subjected to separation in one or more separation apparatus in order to produce the bitumen product and the froth treatment tailings. Exemplary separation apparatus include gravity settling vessels, inclined plate separators and centrifuges.
Some commercial froth treatment processes use naphthenic type diluents (defined as froth treatment diluents which consist of or contain a significant amount of one or more aromatic compounds). Examples of naphthenic type diluents include toluene (a light aromatic compound) and commercial naphtha, which may be comprised of both aromatic and non-aromatic compounds.
Other commercial froth treatment processes use paraffinic type diluents (defined as froth treatment diluents which consist of or contain significant amounts of one or more relatively short-chained aliphatic compounds). Examples of paraffinic type diluents are C4 to C8 aliphatic compounds and natural gas condensate, which typically contains short-chained aliphatic compounds and may also contain small amounts of aromatic compounds.
Froth treatment processes which use naphthenic type diluents (i.e., naphthenic processes) typically result in a relatively high bitumen recovery (perhaps about 98 percent), but also typically result in a bitumen product which has a relatively high solid mineral material and water concentration (also described as “bottom sediment and water concentration” or “BS&W content”).
Froth treatment processes which use paraffinic type diluents (i.e., paraffinic processes) typically result in a relatively lower bitumen recovery (in comparison with naphthenic processes), and in a bitumen product which has a relatively lower BS&W content (in comparison with naphthenic processes). Both the relatively lower bitumen recovery and the relatively lower BS&W content may be attributable to the phenomenon of asphaltene precipitation, which occurs in paraffinic processes when the concentration of the paraffinic type diluent exceeds a critical level. This asphaltene precipitation results in bitumen being lost to the froth treatment tailings, but also provides a cleaning effect in which the precipitating asphaltenes trap solid mineral material and water as they precipitate, thereby separating the solid mineral material and the water from the bitumen froth.
Froth treatment tailings therefore typically contain solid mineral material, water, froth treatment diluent, and small amounts of residual tailings bitumen (perhaps about 2-12 percent of the bitumen which was contained in the original bitumen froth). Much of the froth treatment diluent is typically recovered from the froth treatment tailings in a tailings solvent recovery unit (TSRU). The froth treatment tailings (including the tailings bitumen) are then typically disposed of in a tailings pond. As a result, a significant amount of bitumen from the original oil sand ore is typically lost to the froth treatment tailings as tailings bitumen. There are both environmental incentives and economic incentives for recovering all or a portion of this tailings bitumen.
Canadian Patent Application No. 2,548,006 (Erasmus et al) and corresponding U.S. Patent Application Publication No. US 2007/0272596 A1 (Erasmus et al) describe a process for recovering heavy minerals from oil sand tailings (i.e., froth treatment tailings) in which the tailings are first “deslimed” in a desliming means in order to remove a portion of the free fines and residual bitumen therefrom. The desliming means is comprised of one or more enhanced gravity separators, such as hydrocyclones or centrifuges. The deslimed oil sand tailings are then processed by being sequentially attritioned in an attritioner and separated in a separation means to separate the heavy minerals from other coarse solids present in the deslimed oil sand tailings and produce a concentrated heavy minerals fraction. The attritioner may be a Denver Cell™ type attritioner. The separation means may be comprised of a wide variety of separation apparatus and/or of combinations of such separation apparatus. The concentrated heavy minerals fraction may be further processed to remove residual bitumen therefrom and thereby produce a washed concentrated heavy minerals fraction. No processing is described for the slimes which are removed by the desliming means.
Canadian Patent No. 1,081,642 (Porteous) describes a method for treating froth treatment tailings obtained directly from a dilution centrifuging circuit which comprises introducing the tailings into a flotation cell, subjecting the tailings to agitation and flotation using gas introduced into the base of the body of tailings in order to recover bitumen and diluent as froth and in order to reject a portion of the solids and water as underflow, and removing the froth from further treatment.
Canadian Patent No. 1,094,484 (Lane et al) describes a method similar to the method in Porteous, with the added steps of mixing the froth with a further portion of hydrocarbon diluent, treating the diluted froth in a scroll-type centrifugal separator to reject solids, water and a minor part of the hydrocarbons as tailings and produce a first product stream comprising hydrocarbons, water and a minor part of the solids, and treating the first product stream in a disc-type centrifugal separator to reject water, solids and a minor part of the hydrocarbons as tailings and produce a second product stream comprising hydrocarbons and a minor part of the water and solids.
Canadian Patent No. 1,252,409 (St. Amour et al) describes a method for recovering bitumen from a waste sludge obtained from a retention pond used to store tailings from water extraction of bitumen from tar sands. The tailings comprising the waste sludge are collected from various processing steps of the “hot water” process for primary extraction of bitumen from tar sands. The method includes the steps of conditioning the sludge by removing carbon dioxide and methane and thereafter reducing the viscosity of the sludge, subjecting the conditioned sludge to air flotation in an induced air type of flotation cell in order to obtain a froth, subjecting the froth to a froth settler wherein the mineral tailings are drained off and delivered to a cleaner cell for further processing, diluting the froth from the froth settler with water, deaerating the diluted froth, and separating a bitumen product from the froth. Separating the bitumen product from the froth includes diluting the deaerated froth with hot naphtha and heating the froth, feeding the diluted and heated froth to a hydrocyclone, feeding the overflow from the hydrocyclone to a centrifuge, and recovering the overflow from the centrifuge as the bitumen product.
There remains a need for methods for recovering bitumen from froth treatment tailings.